Scalable Construction for Lateral Semiconductor Components having High Current-Carrying Capacity

ABSTRACT

The invention relates to semiconductor components, in particular to a scalable construction for lateral semiconductor components having high current-carrying capacity. A transistor cell according to the invention comprises a control electrode ( 203 ), a plurality of source fields ( 201 ) and a plurality of drain fields ( 202 ). The control electrode completely encloses at least one of the source fields or drain fields. A transistor according to the invention comprises a plurality of transistor cells on a substrate, each of which comprises a source contact field ( 206 ) and/or a drain contact field ( 207 ). The source contact fields are conductively connected to each other on the other side of the substrate and the drain contact fields are likewise conductively connected to each other on the other side of the substrate. The method according to the invention for producing a transistor comprises the following steps: providing a substrate; forming a plurality of transistor cells on the substrate, each of which comprises a control electrode, a plurality of source fields and a plurality of drain fields; conductively connecting the control electrodes to each other; forming a source contact field and/or a drain contact field in each transistor cell; conductively connecting the source contact fields

The invention relates to a transistor cell according to the preamble of claim 1, to a transistor, in particular to a lateral transistor having a high current-carrying capacity, according to the preamble of claim 10, to a method for producing a transistor according to the preamble of claim 14 as well as to a diode according to the preamble of claim 15.

Due to a contact between a gallium nitride (GaN) layer and an aluminum gallium nitride (AlGaN) layer, gallium-nitride-based transistors may have a highly conductive layer at the AlGaN/GaN interface that can be used as a transistor channel of a field-effect transistor. The conductive layer is contacted via two metal faces that are deposited on the semiconductor surface and represent the source and the drain of the transistor. A control electrode deposited between the source and the drain onto the surface is used as a gate of the field-effect transistor. Such a construction is characterized in that all three transistor terminals are accessible via the top side of the semiconductor and in that the current in the transistor channel flows parallel to the semiconductor surface. Such a transistor construction is referred to as lateral component. In contrast thereto, many transistors (e.g., in silicon technology) are designed as vertical components, wherein at least one transistor terminal, usually the source or the drain, can be contacted from the lower side of the semiconductor substrate.

The development of GaN-based field-effect transistors having a gate made of a Schottky metal (HEMT) or having a gate made of a metal insulated by a dielectric (MISFET) is well advanced for applications as microwave amplifiers. Typically, such components have gate widths of less than 100 mm, wherein the arrangement of the source, drain and gate electrodes on the semiconductor surface is determined by the peculiarities of electric signal propagation in the frequency range of microwaves and results in alternating source fields and drain fields that are arranged one below the other and between which the gate electrodes are arranged. Usually, the gate electrodes are electrically conductively connected to each other on one side thereof at the side of the source fields and drain fields.

The development of GaN-based transistors as switching transistors in the field of power electronics is less advanced. In this field, transistors having a higher pulse-current-carrying capacity (typically, more than 50 A) and having a greater gate width (typically, more than 100 mm) are required. Since the desired switching frequencies in the field of power electronics are much lower than 1 GHz and the length of the electromagnetic waves can thus be considered as being very large as against the transistor dimensions, there is more freedom with regard to the arrangement of the source, drain and gate electrodes on the semiconductor surface.

Thus, it is obvious that the lateral construction of a GaN-based transistor for switching in the field of power electronics may differ from the construction of a GaN-based microwave transistor as well as from the construction of a vertical switching transistor for power electronics. Attention must be particularly turned to the efficient use of the semiconductor surface since the costs per semiconductor surface are particularly high with GaN-based semiconductors.

An object of the present invention is to provide a transistor construction that uses the available semiconductor surface as efficiently as possible and achieves a current-carrying capacity that is as high as possible for a given semiconductor surface. The transistor construction should be structured in such a manner that simple scaling of the current-carrying capacity of the transistors is possible. The electrically active regions should be arranged in such a manner that the temperature of the electrically active regions that develops during transistor operation is distributed on the semiconductor as constantly as possible.

These objects are achieved by a transistor cell with the features mentioned in claim 1, by a transistor with the features mentioned in claim 10 and by a method with the features mentioned in claim 15, respectively.

The inventive transistor cell according to claim 1 comprises a control electrode, a plurality of source fields and a plurality of drain fields. The control electrode completely encloses at least one of the source fields and/or drain fields, whereby an active transistor region is made possible on all edges of the at least one of the source fields and/or drain fields.

While the various construction types presented herein are based on the technology of lateral GaN-based transistors, they are applicable to the same extent to lateral transistors that are based on other semiconductor technologies (e.g., to field-effect transistors and bipolar transistors). Herein, the terminals of general transistors are generally referred to as control electrode, source and drain, which may be, e.g., the base, the emitter and the collector of a bipolar transistor or the gate, the source and the drain of a field-effect transistor, i.e., a control electrode may be, e.g., a base or a gate, a source may be, e.g., an emitter or a source, and a drain may be, e.g., a collector or a drain. Herein, a source field is a region on a semiconductor surface that is designed to function as a part of a source, and a drain field is a region on a semiconductor surface that is designed to function as a part of a drain.

The inventive transistor cell may also have a source contact field and/or a drain contact field, wherein the source fields are conductively connected to the source contact field and/or the drain fields are conductively connected to the drain contact field, wherein a source contact field is a region on a semiconductor surface that is used to establish the contact to one or several source field/s and a drain contact field is a region on a semiconductor surface that is used to establish the contact to one or several drain field/s. If a transistor cell has both a source contact field and a drain contact field, the source fields may be connected to the source contact field and the drain fields may be connected to the drain contact field. On the other hand, if a transistor cell only has a source contact field, the drain fields of the transistor cell may be connectable to drain contact fields of one or several other transistor cell/s. Accordingly, the source fields of the transistor cell may be connectable to source contact fields of one or several other transistor cell/s if the transistor cell only has a drain contact field.

Preferably, the source contact field and/or the drain contact field of the inventive transistor cell has at least one bump that is conductively connectable to a circuit board, thereby achieving that the source fields and/or the drain fields of the transistor cell are connectable to a circuit board as easily as possible, wherein a bump is a structure (e.g., made of solder) that is deposited on a contact field in order to make contacting the contact field from the other side of the semiconductor surface possible.

Preferably, the at least one bump is designed to remove the generated dissipated heat. The control electrode may be concentrically arranged around the at least one bump, thereby minimizing the heat resistance between the active transistor region and the at least one bump so that the at least one bump can be used to remove the dissipated heat of the transistor towards the circuit board in a particularly efficient manner. Preferably, the concentric control electrode and the active transistor region connected thereto are arranged in such a manner that the temperature of the active transistor region that develops during transistor operation does not increase with increasing distance from the at least one bump.

In a preferred embodiment of the inventive transistor cell, the source fields and/or the drain fields are rectangular, thereby achieving a particularly simple surface-filling arrangement. In a further preferred embodiment of the inventive transistor cell, the control electrode has a hexagonal arrangement, thereby achieving that the semiconductor surface can be completely filled with source fields and drain fields with bumps arranged in the center thereof. In still a further preferred embodiment of the inventive transistor cell, the control electrode has a polygonal arrangement, wherein the number of edges of the polygons is an integral multiple of 4, thereby achieving that the source fields and the drain fields can be connected to the source contact fields and the drain contact fields, respectively, via a metal bridge in a particularly simple manner.

The inventive transistor according to claim 10 comprises a plurality of transistor cells on a semiconductor surface, wherein each transistor cell comprises a source contact field and/or a drain contact field. The source contact fields are conductively connected to each other on the other side of the semiconductor surface and the drain contact fields are also conductively connected to each other on the other side of the semiconductor surface, thereby achieving that the high source and drain currents of the interconnected transistor cells do not flow on the semiconductor surface level so that wiring on the semiconductor surface is reduced and current-carrying capacity can be increased. Preferably, each transistor cell has the preferred features described above.

Preferably, the transistor cells on the semiconductor surface are laterally fitted together. Preferably, the bumps of the source contact fields are conductively connected to each other by means of conductive tracks on a circuit board and the bumps of the drain contact fields are also conductively connected to each other by means of conductive tracks on the circuit board. Preferably, the thermal conductivity of the circuit board is high. As against the wire bond connections usually used in the field of power electronics, chip bonding via a bump is characterized by lower inductance, whereby very fast transistors are made possible.

Preferably, the control electrodes of the transistor cells are conductively connected to each other on a substrate that is not the substrate on which the source contact fields and the drain contact fields are conductively connected to each other. In particular, the control electrodes of the transistor cells may be conductively connected to each other on the semiconductor surface level.

In the inventive transistor, the high source and drain currents of the interconnected transistor cells do not flow on the chip level but preferably on a circuit board so that a substantial part of the transistor wiring levels is shifted from the expensive semiconductor surface to a comparatively very cheap circuit board. Moreover, thick copper tracks can be created on circuit boards in a particularly simple manner, which copper tracks have a higher conductivity than (with the same cross-sectional area as) conductive tracks that can be realized on the semiconductor surface by vapor deposition or sputtering.

The individual transistor cells are capable of functioning independently of each other on the wafer level, i.e., prior to the flip-chip mounting of the transistor on the circuit board. This provides the possibility of electrically characterizing the transistor by electrically characterizing its transistor cells, wherein the current-carrying capacity of the used set-up of measuring instruments must only correspond to the transistor cells, fault localization in individual transistor cells is possible, and the straggling of the parameters of individual cells of the transistor can be determined. Moreover, in transistors having a very high number of cells, individual faulty cells can be eliminated by omitting the corresponding bumps in the transistor.

The inventive method according to claim 15 for producing a transistor comprises the following steps: providing a substrate; forming a plurality of transistor cells on the substrate, each of which comprising a control electrode, a plurality of source fields and a plurality of drain fields; conductively connecting the control electrodes to each other; forming a source contact field and/or a drain contact field in each transistor cell; conductively connecting the source fields of each transistor cell to a source contact field; conductively connecting the drain fields of each transistor cell to a drain contact field; forming at least one bump on each of the source contact fields and on each of the drain contact fields; providing a circuit board; conductively connecting the bumps of the source contact fields to each other by means of conductive tracks on the circuit board; and conductively connecting the bumps of the drain contact fields to each other by means of conductive tracks on the circuit board.

A further object of the present invention is to provide a diode construction having the advantages described above.

This object is achieved by a diode with the features mentioned in claim 16. The inventive diode according to claim 16 comprises a plurality of diode cells on a semiconductor surface, wherein each diode cell comprises a cathode contact field and/or an anode contact field. The cathode contact fields are conductively connected to each other on the other side of the semiconductor surface and the anode contact fields are also conductively connected to each other on the other side of the semiconductor surface, thereby achieving that the high cathode and anode currents of the interconnected diode cells do not flow on the semiconductor surface level so that wiring on the semiconductor surface is reduced and current-carrying capacity can be increased.

In the following, exemplary embodiments of the invention will be explained in greater detail on the basis of the associated drawings in which:

FIG. 1 shows a schematic cross-sectional view of an inventive transistor;

FIG. 2 shows a top view of a first exemplary embodiment of an inventive transistor cell;

FIG. 3 shows a top view of a second exemplary embodiment of inventive transistor cells; and

FIG. 4 shows a top view of a third exemplary embodiment of inventive transistor cells.

FIG. 1 shows the cross-section of an inventive transistor by way of example. The transistor is divided into several identical transistor cells 103 on the semiconductor surface 102, which semiconductor surface 102 may be located on a substrate 101. Each individual transistor cell 103 is a transistor capable of functioning. The control electrodes of all transistor cells of the transistor are connected to each other on the semiconductor surface. Different from usual, the sources and drains of the transistor cells are not completely interconnected on the semiconductor surface. Each of the source and drain contact fields 104 of the transistor cells is provided with a bump 105. Moreover, the control electrode contact of the complete transistor is provided with one or several bumps. According to a flip-chip method, the bumps are connected to conductive tracks 106 on a circuit board 107 that represents a circuit board type usually used in electronic construction technology. Preferably, the thermal conductivity of the circuit board 107 is high. The conductive tracks on the circuit board are arranged in such a manner that they connect all source bumps and drain bumps, respectively, in parallel in such a manner that all transistor cells make up a transistor that is connected in parallel and has a high current-carrying capacity. The size of the transistor cells is selected such that their current-carrying capacity is adapted to that of the bump contacts.

FIGS. 2, 3 and 4 show top views of exemplary embodiments of inventive transistor cells. The transistor cells are arranged on the semiconductor surface in the form of a sequence of alternating source fields 201, 301, 401 and drain fields 202, 302, 402 that are separated from each other by a reticulate, coherent control electrode 203, 303, 403 so that an active transistor region 204 on all edges of the source and drain fields is made possible provided that the corresponding field is not located on an outside of the transistor cell. The control electrodes 203, 303, 403 of different transistor cells are electrically conductively connected to each other by metal strips 205, 304, 404 that are arranged on the semiconductor level.

In a first possible embodiment shown in FIG. 2, the source and drain fields 201 and 202 are rectangular. The source contact fields 206 and drain contact fields 207 with the bumps 208 are located on opposite sides of the field with the active transistor regions. The electrically conductive connection between the source and drain fields and the associated contact fields is realized by means of metal bridges that may be realized in the form of air bridges or of bridges over a dielectric, which dielectric is open over the source or drain fields to be contacted.

In a second possible embodiment shown in FIG. 3 and a third possible embodiment shown in FIG. 4, the source and drain fields 301, 401 and 302, 402, respectively, are concentrically arranged around a source or drain contact field 305, 405 and 306, 406, respectively, with a bump 307, 407. The control electrodes 303, 403 make up a network of concentric and radial metallization elements that are electrically conductively connected to the control electrodes of the remaining transistor cells of the transistor by means of metal strips 304, 404 that are arranged on the semiconductor level. The advantage of the embodiments shown in FIGS. 3 and 4 consists in the minimized heat resistance between the active transistor region and the bump that can be used to remove the dissipated heat of the transistor towards the circuit board in a particularly efficient manner. Moreover, the concentric arrangement of the control electrodes enables the distance between them to be increased with increasing distance from the bump in such a manner that the temperature on the semiconductor surface that develops during transistor operation does not increase with increasing distance from the bump. The source and drain fields are contacted in the same manner as in the first embodiment described above, wherein the source fields that are located in transistor cells that have a source contact field are connected to this source contact field, and the source fields that are located in transistor cells that have a drain contact field are connected to the source contact field of an adjacent transistor cell. The same applies to the drain fields.

If the arrangement of the control electrodes is hexagonal (as shown with four transistor cells in FIG. 3), the semiconductor surface can be completely filled with source and drain fields with source or drain bumps 307 arranged in the center thereof.

If the arrangement of the control electrodes is octagonal (as shown with four transistor cells in FIG. 4), a transistor cell having a central source contact field 405 is surrounded on all sides by transistor cells having a central drain contact field 406, and vice versa. The advantage of this arrangement consists in the fact that the individual source and drain fields 401 and 402 can be connected to the source and drain contact fields 405 and 406 via a metal bridge in a particularly simple manner.

The advantages described with respect to the octagonal embodiment shown in FIG. 4 apply to all bump-centered polygonal embodiments in which the number of edges of the polygons is an integral multiple of 4. Other embodiments of the lateral arrangements of the source and drain fields with the intermediate control electrodes are possible.

Each of the embodiments described above can also be applied to an inventive diode construction. To this end, it is necessary in each case to omit the control electrodes and their contacts and to replace the source fields with cathode fields, to replace the source contact fields with cathode contact fields, to replace the drain fields with anode fields, and to replace the drain contact fields with anode contact fields. The cathode contact fields are conductively connected to each other on the other side of the semiconductor surface to form a cathode, and the anode contact fields are conductively connected to each other on the other side of the semiconductor surface to form an anode. This results in a diode that has a high current-carrying capacity and efficiently uses the semiconductor surface.

LIST OF REFERENCE NUMERALS

101 substrate

102 semiconductor surface

103 transistor cell

104 source contact field/drain contact field

105 bump

106 conductive track

107 circuit board

201, 301, 401 source field

202, 302, 402 drain field

203, 303, 403 control electrode

204 active transistor region

205, 304, 404 metal strip

206, 305, 405 source contact field

207, 306, 406 drain contact field

208, 307, 407 bump 

1. A transistor cell, comprising: a control electrode; a plurality of source fields; and a plurality of drain fields, wherein the control electrode (203, 303, 403) completely encloses at least one of the source fields and at least one of the drain fields.
 2. A transistor cell according to claim 1, wherein the transistor cell further has a source contact field and/or a drain contact field, wherein the source fields are conductively connected to the source contact field and/or the drain fields are conductively connected to the drain contact field.
 3. A transistor cell according to claim 1, wherein the source contact field and/or the drain contact field has at least one bump that is conductively connectable to a circuit board.
 4. A transistor cell according to claim 3, wherein the at least one bump is designed to remove the generated dissipated heat.
 5. A transistor cell according to claim 3, wherein the control electrode is concentrically arranged around the at least one bump.
 6. A transistor cell according to claim 5, wherein the concentric control electrode is arranged in such a manner that the temperature of the control electrode that develops during transistor operation does not increase with increasing distance from the at least one bump.
 7. A transistor cell according to claim 1, wherein the control electrode has a rectangular arrangement.
 8. A transistor cell according to claim 1, wherein the control electrode has a hexagonal arrangement.
 9. A transistor cell according to claim 1, wherein the control electrode has a polygonal arrangement, wherein the number of edges of the polygons is an integral multiple of
 4. 10. A transistor having a plurality of transistor cells on a semiconductor surface, wherein each transistor cell comprises a source contact field and/or a drain contact field, wherein the source contact fields are conductively connected to each other on the other side of the semiconductor surface; and the drain contact fields are conductively connected to each other on the other side of the semiconductor surface, wherein each transistor cell is a transistor cell according to claim
 1. 11. A transistor according to claim 10, wherein the transistor cells on the semiconductor surface are laterally fitted together.
 12. A transistor according to claim 10 having transistor cells wherein the source contact fields and/or the drain contact fields have at least one bump that is conductively connectable to a circuit board; and wherein the bumps of the source contact fields are conductively connected to each other by means of conductive tracks on a circuit board; and the bumps of the drain contact fields are conductively connected to each other by means of conductive tracks on the circuit board.
 13. A transistor according to claim 10, wherein the control electrodes of the transistor cells are conductively connected to each other on a substrate that is not the substrate on which the source contact fields and the drain contact fields are conductively connected to each other.
 14. A method for producing a transistor, comprising: providing a semiconductor surface; forming a plurality of transistor cells on the semiconductor surface, each of which comprising a control electrode, a plurality of source fields and a plurality of drain fields; and conductively connecting the control electrodes to each other, forming a source contact field and/or a drain contact field in each transistor cell; conductively connecting the source fields to one source contact field each; conductively connecting the drain fields to one drain contact field each; forming at least one bump on each of the source contact fields and on each of the drain contact fields; providing a circuit board; conductively connecting the bumps of the source contact fields to each other by means of conductive tracks on the circuit board; and conductively connecting the bumps of the drain contact fields to each other by means of conductive tracks on the circuit board, wherein in each of the plurality of transistor cells, the control electrode is formed in such a manner that it completely encloses at least one of the source fields and at least one of the drain fields.
 15. A diode having a plurality of diode cells on a semiconductor surface, wherein each diode cell comprises a cathode contact field and/or an anode contact field, wherein the cathode contact fields are conductively connected to each other on the other side of the semiconductor surface; and the anode contact fields are conductively connected to each other on the other side of the semiconductor surface. 